1. Field of the Invention
This invention relates to a process and formulation for producing fuel pellets or briquettes from fine sized coal and waste plastic, and to an improved fuel pellet made from fine sized coal and waste plastic.
2. Brief Description of the Prior Art
It has been estimated that over two billion tons of fine sized coal, i.e., coal having particle sizes of under about 0.5 mm, is contained in abandoned or active slurry impoundments in the U.S. While cleaning technologies exist to reduce the ash content of such coal fines to an acceptable level for fuel use, large quantities of such fines are still discarded because major users such as electric utility generating plants and industrial boiler operations place restrictions on the amount of fines in the coal that they purchased. The primary reason for these restrictions is that the fines normally contain a high percentage of moisture because of their high surface to weight ratio. The high moisture content results in serious handling problems because of the coal's stickiness, and its tendency to freeze in cold climates. If the fines are thermally dried, dust problems result and normal precipitation may quickly rewet the fines stored outdoors.
It is known that the problems in handling and burning coal fines can be reduced if the coal is pelletized. However, pelletizing coal fines has normally required use of an adhesive binder, or the use of very high forming pressure and/or high temperatures. As is known, coal particles do not naturally stick together unless heated to the point of becoming plastic, or about 650.degree. F. for most coals, or unless the particles are compressed at extremely high pressures, normally over 20,000 PSI for most coals. Each of these alternatives are expensive and may present other problems. For example, heating coal to 650.degree. F. or higher can cause evolution of volatiles contained in the coal and of course the pellets must be cooled at least to some extent before storage in bulk.
It is also well known that large quantities of used plastic materials, both recyclable and nonrecyclable, are landfilled in the U.S. and elsewhere. This plastic material has a relatively high heating value, suggesting its use as a fuel. Such plastic material can be successfully burned as a fuel in boilers especially designed for such use, but such boilers are expensive both to build and to operate. It is also known that plastics may be blended with coal before combustion in a conventional boiler type furnace, but the plastic often segregates during storage and handling thereby causing nonhomogeneous fuel mixtures to be fed to the boiler.
Since the heat content and ash characteristics of plastic and coal are very different, slugs of plastic (or coal) can cause wide variations in boiler efficiency and in stack emissions. While commercially available pulverizers are available for shredding or pulverizing waste plastics, large volumes or slugs of such plastics may cause problems in boilers. Also, such plastic material typically is more difficult to handle than coal, potentially causing plugging in chutes and bridging or plugging in bins.
It is known to produce compact fuel pellets, briquettes, or even synthetic fireplace logs from a blend or mixture of combustible materials including coal fines, cellulosic materials including waste paper and paper making sludge, sewage sludge, plastics, and other materials. While these materials may be satisfactory for certain uses, they generally have had certain drawbacks which limit their commercial utilization. For example, the compaction process, hereinafter generally referred to as briquetting, may be expensive and the compacts produced may lack sufficient strength for bulk handling and transportation, or may be insufficiently water resistant to permit outdoor bulk storage.
Numerous patents have issued on fuel pellets or fuel pelletizing processes, in which the pellets are formed from or include coal fines as a substantial portion of the final product. One such product and process is disclosed in U.S. Pat. No. 5,743,924 to Dospoy et al, assigned to the assignee of the present application. This patent discloses a fuel pellet, and its method of manufacture, in which the pellet contains coal fines in the amount of 60 to 80%, papermaking sludge in an amount of 10 to 30%, and low density polyethylene plastic film in the amount of 0.5 to about 15% by weight. The process involves shredding the plastic film, blending the mixture, and pelletizing at a pressure within the range of about 1,000 to 4,000 PSIG. The patent also discloses the use of about 5 to about 10% by weight of paper in addition to the paper-making sludge. While the process of this patent is in commercial use and the pellets are satisfactory for certain uses, the pellets do not possess the water resistance desired for prolonged external bulk storage or the strength desired to enable repeated mechanized handling for economical transportation.
Johnston et al U.S. Pat. No. 4,529,407 discloses an injection molded fuel pellet composed of 97 to 99% combustible material of which at least 1/2 is natural cellulosic material with the balance being a filler which could include coal in amounts not in excess of 30%, and from 1 to 3% plastic. The synthetic thermoplastic material is distributed throughout the fuel pellet as discrete particles. The pellet is formed in an extruder where the temperature of the pellet is controlled to assure softening of the plastic within the forming pellet without conglomeration. The synthetic thermoplastic material acts to mechanically bond the cellulosic particles together.
Sardessai et al U.S. Pat. No. 5,244,473 discloses a fuel briquette consisting of particles of coal, coke, or lignite bonded into briquettes by mixing the particles with a phenolformaldehyde resin and a polyisocyanate in the presence of an organic nitrogen containing catalyst, with the mixture being subjected to a briquetting process to form the phenolic-urethane polymer bonded and coated briquettes.
Romey et al U.S. Pat. No. 3,836,343 teaches the formation of smokeless carbon-containing briquettes wherein the smokeless carbonaceous material is mixed with an aqueous suspension of a copolymer of butadine-acrylonitrile and compressed to form smokeless briquettes.
U.S. Pat. Nos. 5,487,764, 5,453,103 and 5,599,361, all assigned to COVOL Technology, each disclose the use of a polymeric material as a binder in the formation of briquettes or pellets of carbonaceous material such as coke breeze, coal fines or revert material.
Of the prior art patents, the Dospoy et al patent and Johnston et al patent each employ a substantial portion of cellulosic material in addition to the coal fines. The Sardessai patent and the three patents assigned to COVOL, mentioned above, all employ relatively expensive thermoplastic materials as binders to react with the coal or other carbonaceous products. With the exception of the Dospoy et al patent, the amount of synthetic resin material employed in the final product is not sufficient to materially affect the final product as a fuel.
It is the primary object of the present invention to provide an improved method of forming a fuel pellet, utilizing only coal fines and waste plastic material to produce a high strength durable, water resistant fuel pellet suitable for use in coal fired furnaces or boilers.
Another object is to provide such a fuel pellet product suitable for burning in conventional coal fired furnaces or boilers and which avoids the disadvantages of burning coal fines or plastic alone, or a mixture of coal fines and plastics.
Another object is to provide such a fuel pellet which is economical to produce and which provides enhanced burning characteristics including an increased heating value, reduced ash content and reduced sulfur content over coal alone.
Another object is to provide an improved alternative to landfill disposal of waste plastics and to the utilization of waste coal fines currently contained in slurry impoundments.
Another object of the invention is to provide such an improved fuel pellet which may be produced without use of the extremely high pelletizing pressures and/or temperatures required for pelletizing coal alone.
Another object is to provide such an improved fuel pellet which avoids the necessity for use of high cost adhesive materials for bonding the coal fines in the pellet.